Making a button's placement independent of resolution

I am extremely new to Unity.

I have a button which I created using Gui.button(...) function but want to place it slightly to the left of the middle of the screen, independent of screen resolution. So I want the button's right side to be touching the center y-axis of the screen and the button to be resting on the x-axis.

How can I do this?

E.g.

 Gui.button(Rect (20,40,80,20), "Level 1"))

is my button, should I set the x position of th button to

 x = Screen.Width/2 - (80-20)/2

Thanks.